Group III nitride semiconductors composed of a compound of N and group III elements such as Al, Ga, and In have conventionally been used as material of light-emitting elements from blue light to deep ultraviolet light. In particular, group III nitride semiconductors composed of AlGaN with a high Al composition have been used in ultraviolet light-emitting elements with an emission wavelength of 340 nm or less and deep ultraviolet light-emitting elements (DUV-LED) with an emission wavelength of 300 nm or less.
It has been commonly considered that a deep ultraviolet light-emitting element using a group III nitride semiconductor has very low light extraction efficiency and so is difficult to achieve higher output. However, to provide a small and high-output deep ultraviolet light-emitting element, various attempts to achieve high external quantum efficiency, low resistance, and the like have been made.
For example, we previously proposed the following light-emitting element in JP 2010-205767 A (PTL 1). A light-emitting element formed on an AlN-based group III nitride single crystal includes: a high-concentration n-type group III nitride layer; a multiple quantum well structure composed of an n-type or i-type group III nitride barrier layer and an n-type or i-type group III nitride well layer; an i-type group III nitride final barrier layer; a p-type group III nitride layer; and an electron block layer formed between the i-type group III nitride final barrier layer and the p-type group III nitride layer and composed of a p-type or i-type AlzGa1-zN layer (0.95<z≤1) that serves as an electron energy barrier for the i-type group III nitride final barrier layer.
With the technique described in PTL 1, internal quantum efficiency is improved by optimizing the quantum well thickness of the quantum well structure, and electron injection efficiency is optimized by introducing and optimizing the electron block layer and optimizing the final barrier layer.